Field of the Invention
The invention relates to a memory cell array having memory elements with magnetoresistive effect and a method for manufacturing it.
Technologie Analyse XMR-Technologien, Technologie-früh-erkennung [Technology Analysis XMR Technologies, Early Recognition of Technology], by Stefan Mengel, Publisher VDI-Technologiezentrum Physikalische Technologie, discloses layered structures with magnetoresistive effect. Depending on its design, the layered structure is classified as a GMR (giant magnetoresistance) element, TMR (tunneling magnetoresitive) element, AMR (anisotropic resistance) element or CMR (colossal magnetoresistance) element. The term GMR element is used in the art to designate layered structures which have at least two ferromagnetic layers and a nonmagnetic, conductive layer arranged between them and exhibit the GMR (giant magnetoresistance) effect, that is to say exhibit a large magnetoresistive effect in comparison with the AMR (anisotropic magnetoresistance) effect. The GMR effect is understood as referring to the fact that the electrical resistance of the GMR element is dependent on whether the magnetizations in the two ferromagnetic layers are oriented in parallel or antiparallel both for currents which are parallel (CIP current in plane) and perpendicular (CPP current perpendicular to plane) to the layer planes. The resistance changes here as a function of the orientation of the magnetizations by ΔR/R=5 percent to 20 percent at room temperature.
The term TMR element is used in the specialist field for “Tunneling Magnetoresistance” layered structures which have at least two ferromagnetic layers and an insulating, nonmagnetic layer arranged between them. The insulating layer has such a small thickness that a tunnel current occurs between the two ferromagnetic layers. These lead structures also exhibit a magnetoresistive effect which is brought about by spin-polarized tunnel current through the insulating, nonmagnetic layer arranged between the two ferromagnetic layers. In this case also, the electrical resistance of the TMR element (CPP arrangement) is dependent on whether the magnetizations in the two ferromagnetic layers are oriented in parallel or antiparallel. The resistance varies by ΔR/R=10 percent to approximately 30 percent at room temperature.
The AMR effect is due to the fact that the resistance in magnetized conductors parallel to the magnetization direction varies from that of magnetized conductors which are perpendicular to the magnetization direction. It is a volume effect and thus occurs in ferromagnetic single layers.
A further magnetoresistive effect which is referred to as colossal magnetoresistance effect due to its magnitude (ΔR/R=100 percent to 400 percent at room temperature) requires a high magnetic field for switching between the magnetization states owing to its high coercitive forces.
It has been proposed (see for example D. D. Tang, P. K. Wang, V. S. Speriosu, S. Le, K. K. Kung, “Spin Valve RAM Cell”, IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1996, page 3206) to use GMR elements as memory elements in a memory cell array. The magnetization direction of the one ferromagnetic layer of the GMR element is held here, for example, by an adjacent antiferromagnetic layer. Intersecting x and y lines are provided. In each case a memory element is arranged at the points of intersection of the x/y lines. In order to write information, the x/y lines are supplied with signals which bring about at the point of intersection a magnetic field which is sufficient for the change of polarity. In order to read out the information, the x/y lines can be supplied with a signal which switches the respective memory cell to and fro between the two magnetization states. The current through the memory element from which the resistance value, and thus the information, is determined is measured.
In order to write and read, local magnetic fields of 10 Oe to approximately 100 Oe corresponding to 8 A/cm to 80 A/cm are necessary. It is desirable here for the magnetic fields to be generated by the smallest possible current in the lines.
However, as miniaturization progresses, the current densities necessary to generate the local magnetic fields become greater. In addition, an effect has been observed (see M. H. Kryder, Kie Y. Ahn, N. J. Mazzeo, S. Schwarzl, and S. M. Kane, “Magnetic Properties and Domain Structures in Narrow NiFe Stripes”, IEEE Transactions on Magnetics, Vol. Mag.-16, No. 1, January 1980, page 99), in which the magnetic switching field thresholds increase as the dimensions become smaller, that is to say higher currents become necessary for switching.